Cellular radio communication systems and methods and equipment for use therein

ABSTRACT

Disclosed is a method of operation for use in a radio communications system comprising a cellular network of base stations and mobile units linked to the base stations. The method comprises detecting that a base station has become isolated from the system, wherein the isolated base station has lost at least one link to other base stations but is still in radio communication with its mobile units and changing an output power level of said isolated base station relative to an output power level of one or more selected co-channel base stations of neighboring cells of the cellular system, whereby the range of communications provided by said isolated base station is changed to a different finite value relative to that provided by the said co-channel base stations.

This application claims the benefit of prior filed co-pendinginternational application Serial No. PCT/EP02/03093 filed Mar. 18, 2002,and assigned to Motorola, Inc., which was published by the InternationalBureau on Nov. 14, 2002 under No. WO 02/091622A1; and Great Britainapplication Serial No. 0111317.4 filed May 9, 2001.

FIELD OF THE INVENTION

This invention relates to cellular radio communication systems andmethods and equipment for use therein. More particularly, the inventionrelates to systems comprising a cellular network of base stations eachserving mobile units in its location and a method for maintaininginterconnection between mobile units in the event of accidentalisolation of one or more of the base stations from the system.

BACKGROUND OF THE INVENTION

A cellular radio communication system, such as illustrated in the blockdiagram of FIG. 1 of the accompanying drawings, generally consists of anarray of base transceiver stations 12 that are deployed over the area ofcoverage of the system and connected via a network of broadband links 14to each other and to a central switch or controller 16. Each basestation 12 sends radio signals (over a downlink 17) to, and receivesradio signals (over an uplink 19) from, mobile transceiver units 18(also to be referred to as subscriber units or subscribers) locatedwithin an area surrounding it, or within a sector thereof, termed acell. The mobile transceiver units 18 may be mobile telephones, mobiledata communication units or portable or vehicle mounted radio units orthe like. The size of a cell (e.g. its radius about the base station) isdefined as that over which a usable signal is received from therespective base station and largely depends on the power at which itstransmitter operates downlink (to be referred to, for brevity, as the“power” of the base station); this subject is discussed further below.The exact siting and programmed power of each base station are planned,according to, inter alia, the topography of the area, so as to providecontinuous coverage by the cells, with overlapping regions. Each basestation is assigned a set of physical channels, over which itcommunicates with the mobile subscriber units within its cell. Such aset of channels is usually characterized by a group of unique carrierfrequencies, but may also be distinguished by other multiplexing methodsknown in the art, such as CDMA (code division multiple access). There isa finite number of such channel sets and these are assigned to thevarious base stations so that, at the very least, no adjacent cellsshare a channel set and preferably so that cells or base stationssharing a channel set, termed co-channel cells or co-channel basestations, are mutually distant sufficiently to keep mutual interferencebelow some given threshold value. An ideal layout (which may arise in asituation of homogenous topography and requirements) is schematicallydepicted in FIG. 2, as an illustrative example. Here the base stationsand their associated cells are arranged in a regular hexagonal cell gridand they are assigned seven sets of channels. A channel re-use patternin a network of cells in a system operating as shown in FIG. 1 is thusshown in FIG. 2. The numerals in the cells in FIG. 2 indicate use ineach cell of a particular channel, e.g. frequency, in the set of sevenchannels numbered 1 to 7. In a practical network, the base stations aregenerally deployed in a much less regular manner and the shapes andsizes of their associated cells will vary. The present invention isequally applicable to any system, having any cell size and shape and anydeployment topology.

Generally each mobile unit can receive signals from several proximatebase stations. Each received signal is characterized by acarrier-to-noise ratio (C/N) and a carrier-to-interference ratio (C/I),where the interference is mainly from nearby co-channel base stations.Both ratios depend on the current downlink power of the correspondingbase station; the C/I, however also depends on the current power of thenearby co-channel base stations. These ratios can be measured by themobile subscriber unit and compared against each other, as well as withstored threshold values. A mobile unit is generally programmed initiallyto scan the channels (e.g. the frequencies) and to select that with thehighest C/I and/or highest C/N and then to establish communication withthe corresponding base station, which thus becomes the so-called activebase station. The mobile subscriber unit is then said to be linked tothe active base station. When any of the two ratios drops below acertain respective threshold level (owing to changing location or othercauses), in some types of systems the power of the base station at therespective channel is raised and if it has reached its maximumprogrammed value, which in other systems (notably those based onTDMA—time divison multiple access) is always the case, a so-calledreselection process is initiated, whereby the mobile unit selects fromamong other received signals the one with the highest C/I and/or C/N,switches communication to the corresponding channel and thus becomeslinked to the corresponding base station, which becomes its active basestation. In effect, the mobile subscriber unit thus moves from one cellto another (usually adjacent) cell. If all mobile units have the samethreshold values stored, they would generally undergo reselection, uponmoving away from the active base station, at about the samedistance—which defines the effective range (or just “range”) of the basestation or the boundary (and hence, size) of the cell. Thus, clearly,the effective boundary of any cell is a function of the stored thresholdvalues, as well as of the power of the active base station and of anyinterfering co-channel base stations for each channel. For any system,threshold values and (as mentioned above) also the base stationsdeployment topology and power values are generally chosen so that theareas within the effective boundaries of adjacent cells (i.e. the rangesof their corresponding base stations) overlap substantially, in order tohave sufficient margin of safety for contiguous service.

It is to be noted that the total power transmitted by any base stationis the sum of the powers of all active channels and is limited by thecapacity of the transmitter's power amplifier. In general, the powerlevels of all channels (or their maximum values, in systems where thelevels vary according to reception conditions) in any one base stationare set so that the total power is below the amplifier's capacity—tocorrespond with the designed range of the respective cell. This settingof power levels (whether of individual channels or of the overall power)is usually achieved through appropriate programming and thus can, inprinciple, be changed—e.g. when various conditions change; such a changeis, however, not a common practice and is generally done only throughintentional directed programming, as part of a system re-design.

Typically, a mobile unit has a programmable digital controller, whichstores various parameters related to the communication process,including, in particular, those related to monitoring and calculatingC/N and C/I ratios of received signals, identifying adjacent cells andcontrolling the reselection process. These parameters can be downloadedfrom the system over a control channel during system operation.

A communication path between any two mobile units 18 within a cellularsystem usually consists of the radio link between each of them and itsrespective base transceiver station 12 and the path through thewide-band network 14 between each of the base stations 12. Each radiolink between a base station 12 and a mobile unit 18 consists of thedownlink 17 and the uplink 19. If the two mobile units are within thesame cell, the link may consist of their radio links only, if the basestation is so programmed, in which case they are said to be directlyconnected. One method for such direct connection, for example, is theuse of so-called site trunking protocols. Another method, to beexplained in the next following paragraph, is aimed at group calls.

Each base station 12 typically includes a digital controller 11 and adigital program store 13 (for software programs and data), whichtogether serve to control the communication process between the linkedmobile units 18 and other base stations and the central switch 16 (andwhere appropriate with agents external to the system). Similarly, thecentral switch 16 includes a digital program store 15, to serve incontrolling communication with all of the base stations 12 and with theexternal world beyond the system network.

The present invention is applicable to any type of cellularcommunication system, but it is particularly suitable for use in theso-called Private Mobile Radio system (PMR) or Trunked Mobile Radio(TMR) system—especially when having provision for group calls. In such asystem, which generally serves for dispatch mode of service (i.e. readycommunication within a “fleet” of mobile subscriber units, usuallybelonging to one organization), the users share common channelresources, and are directed to use these resources under the control ofa central controlling entity; the latter is usually the central switchof the system, but in cases such as addressed by the present inventionit may be a fallback base station. During a group call, a defined groupof mobile subscriber units (usually a subset of the fleet) isinterconnected so that all their operators can listen simultaneously toany one of them talling. In many cases, mobile subscriber units definedto belong to a group are frequently located within a relatively smallgeographic area (for example, members of a police unit patrolling atown). Often this area is, to a great extent, congruent with acell—sometimes by design of the system. A group call between mobilesubscriber units linked to the same base station is generally carriedout by direct connection (i.e. not through the central switch)—usuallyby means of a shared channel; this is a semi-duplex mode ofcommunication, whereby all members of the group within the cell use thesame channel pair, one member doing the talking over the uplink channeland all others listening over the downlink channel. Communication withgroup members in other cells is carried out over the usual path, whichincludes the central switch, where the appropriate connections are made.

For normal operation of the cellular network 14 shown in FIG. 1, eachbase station 12 is connected either by radio communication and/or by aphysical line connection to other terminals, especially other basestations 12. Each base station 12 is thereby connected directly orindirectly to the central switch 16 as indicated in FIG. 1. In suchnormal use, network control signals to and from the central switch aswell as traffic signal (signals representing speech, data etc) are sentvia this network of links.

Among the faults that may accidentally occur in an operational system,two give rise to problems which are solved by the present invention. Thefirst one occurs when a base station loses its link to one or more otherbase stations and becomes partially or totally isolated. In consequence,the base station may also become disconnected from the centralcontroller or switch. In such a case, all mobile units currently servedby this base station may continue to be served by this station or, ifappropriate, may communicate with other mobile units within rangedirectly. However, indirect connections to mobile units served by otherbase stations normally connected to the isolated base station via thelost link(s) will be lost.

The second operational fault may occur when all, or a large number of,base stations of the network become isolated from one another or fromthe network infrastructure, for example owing to a fault in the networkor in the central switch. In this case, of course, all inter-cellcommunications will be lost and, again, only intra-cell or directcommunications will be possible.

In many installed PMR or TMR systems, large groups of subscribers, withtheir respective mobile subscriber units, are often located withinrelatively small geographic regions (e.g. towns), with most of thecommunication traffic occurring within any group. Much of this trafficmay be in the form of group calls. Such a region is typically covered bya few contiguous cells and each mobile subscriber unit is serviced bythe base station providing the best signal. If one of these basestations becomes isolated, a so-called fallback situation is declaredand a fallback procedure is initiated. The main objective now becomes tokeep as many subscribers as possible interconnected—which, in mostcases, means keeping as many groups as possible intra-connected. One oftwo general situations may be discerned: (a) one or more groups arelargely located within the isolated cell or immediately adjacent to it;(b) most subscribers within the isolated cell are not interconnected anddo not belong to a common group.

There is thus a clear need for a method and system that will furtherincrease the number of mobile units that remain interconnected infallback situations caused by individual or massive isolation of basestations from the cellular system.

GB-A-280570A describes a procedure for dealing with the isolation of acell caused by failure of its base station whereby service to mobileunits within the cell is lost, i.e. it is assumed that the output fromthe affected base station is lost. This procedure involves expanding therange of adjacent cells by increase of their output power. The procedureis intended to provide communication coverage to some of the mobileunits which have lost service from the failed base station. Byimplication, the cells adjacent to the one which has failed are on adifferent frequency to provide this bordering coverage.

SUMMARY OF THE INVENTION

The present invention is concerned with providing an improved method ofoperating a cellular communication system. The system maybe a digitalcellular radio communication system and may be a system wherein adjacentcells within a given group operate at different frequencies and thefrequencies and preferably the frequency pattern from cell to cell usedwithin the group are repeated in other groups of the network. It is anobject of the present invention to provide in such a system a method forincreasing the number of mobile units remaining in mutual communicationin the event that a base station of one of the cells becomes isolatedfrom the system, owing to a failure in its linkage with the networkcontroller or switch, as well as in the event that a large number ofbase stations become mutually isolated, owing to system failure.

Such an object is realised in accordance with the invention by a methodwhich includes (i) detecting a failure condition relating to the givenbase station whereby one or more links of the base station to other basestations of the system is or are lost and, in consequence; and (ii)changing the output power level of one or more neighbouring basestations; and is characterised in that in the failure condition thegiven base station is partially or totally isolated from other basestations but is still in radio communication with its served mobileunits and the finite output power level of the given isolated basestation and the finite output power level of one or more selectedco-channel base stations of neighbouring cells of the cellular systemare changed relative to one another, whereby the range of communicationsprovided by the given base station is changed to a different finitevalue relative to that provided by the said co-channel base station orstations.

Thus, a fallback mode of operation is initiated such that some mobileunits served by the isolated base station may remain in mutualcommunication with the isolated based station.

More particularly, in the event that a single base station becomesisolated from the system, a first form of the method is as follows. Thisform may be applied as a fallback procedure in the situation where theisolated base station is designated by the system to be favoured, e.g. aconcentrated group of mobile units exists or is expected to exist nearto that base station and/or that base station provides a relatively widearea coverage compared with neighbouring co-channel stations which arebase stations operating on the same frequency as the isolated station.The favoured designation may have been previously set during systemdesign and implementation or it may be detected by the system on theoccasion when the isolation event occurs. This form of the methodincludes increasing the output operational power level of the isolatedbase station, whilst possibly also decreasing the output operationalpower level of neighbouring co-channel base stations. This form of themethod increases C/I ratio over the coverage area of the isolated basestation, thus extending the coverage area of the isolated base stationand this facilitates more mobile units remaining linked to this basestation or switching over to this base station from co-channel basestations of neighbouring cells.

A second form of the method may be applied as a fallback procedure inthe situation where the isolated base station is not designated by thesystem to be favoured, i.e. the number of mobile units linked to thestation is or is expected to be lower than the number linked toneighbouring co-channel stations and/or the area of coverage provided bythe station is smaller than that provided by neighbouring co-channelstations. The unfavoured designation may have been previously set duringsystem design and implementation or it may be detected by the system onthe occasion when the isolation event occurs. The second form of themethod includes decreasing (including where appropriate reducing tozero) the output operational power level of the isolated base station,whilst possibly also increasing the output power level of neighbouringco-channel base stations. This decreases the C/I ratio of the isolatedstation and shrinks the coverage area of the isolated stationfacilitating mobile units to switch over, or stay linked, toneighbouring base stations.

A third form of the method of the invention may be applied as a fallbackprocedure in the situation where many or all of the base stations becomeisolated, e.g. owing to a system failure. This form includes havingcertain base stations designated as key stations, according to theirexpected higher degree of common coverage of mobile units, and havingother base stations, having a lower expected degree of coverage,designated as non-key stations. This form of the method furtherincludes, in the event of isolation of a large number of the basestations, e.g. due to a network failure, increasing or at leastmaintaining the output operational power level of all key stations,whilst possibly also decreasing or switching off the power level ofother base stations, designated as non-key stations. The range of thekey stations is thereby extended and this facilitates more mobile unitsbeing allowed to link, or remain linked, to key base stations.

The method of the present invention may include use of an automaticprocedure programmed into the control system of the base stations and/orthe network controller or switch during system setup, to effect thespecified changes which are required following sensing of an isolationevent. Such procedures are described further below.

The method according to the invention may include, in the case of eachbase station which has changed output power level as a result of theisolation event, sensing that the isolation event has finished andreturning the base station to the output power level existing duringnormal operation before the isolation event.

The method according to the invention may be enhanced when it iscombined with the use of one or more known algorithms which may beprogrammed into mobile units to determine which base station(s) theunits should select to communicate with. The mobile units may beprogrammed such that on sensing of an isolation event the communicationof the mobile unit is steered toward one or more base stations thenfavoured by the mobile unit and/or as directed by the system.

The method of the invention is further defined in the appended claims.

According to the present invention in another aspect there is provided abase station for use in a cellular communication system using theoperational method of the invention, the base station being operable,upon becoming isolated from the system or upon a nearby co-channel basestation becoming isolated from the system, to change its own outputoperating power level, in accordance with the said method, e.g. in oneof the forms defined above.

The change in power level following an isolation event may increase ordecrease the output operational power level of radio communicationtransmissions from the base station as described earlier. The powerlevel that is changed may be the programmed power level in the case of abase station normally having a fixed output power level or it may be themaximum output power level in the case of a base station that normallycan transmit at different power levels.

The need for a base station which becomes isolated to make an outputpower level change can be sensed internally by the base station by lackof signals from the network system controller or switch. In such acondition, an internal controller within the operating control system ofthe base station may be activated to generate automatically a controlsignal which causes the operating power level to be changed, i.e. eitherto be increased or decreased as appropriate. Data may be stored in amemory store within the operating control system of the base stationdefining what the output power level to be provided by the base stationis to be in the event of the base station becoming isolated as indicatedby a signal provided by the internal controller. The power level may beset accordingly by the output power controlling part of the operatingsystem control of the base station.

The need for a base station which remains connected to the network tomake a power change because of isolation of a neighbouring co-channelbase station can be provided by a control signal from the network systemcontroller or switch. Such a condition may be sensed by the systemcontroller or switch by lack of signals from the isolated base station.The control signal sent to the base station by the network controller orswitch can be a simple signal instructing the base station to changepower level, in which case the base station may be pre-programmed torecognise and understand the control signal. The base station maythereby automatically change to a certain pre-defined output powerlevel, which may where appropriate be a zero output power level. Thecontrol signal could alternatively be one of a number of differentpossible signals each giving different instructions. For example, thecontrol signal may indicate that the base station should increase ordecrease its output power level or indicate the power level to which thebase station should change. In this case, the base station will bepre-programmed to recognise the received control signal and to actaccordingly, the required change being effected by the output powerlevel control within the base station internal system control.

According to the present invention in another aspect there is provided acellular communication system for maintaining radio communicationbetween mobile communication units via a cellular network of basestations, wherein the system is operable according to the method of theinvention and wherein each of said base stations is operable to changeits own operating power upon that station or a nearby co-channel stationbecoming isolated from the system in accordance with the said method.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a typical known cellularcommunication system.

FIG. 2 is a schematic diagram of a known idealised layout of contiguouscells in a cellular communication system, showing channel re-usepatterns; the channels are numbered 1 to 7;

FIG. 3 is a diagram similar to that of FIG. 2, schematicallyillustrating a layout of contiguous cells as present in an embodiment ofthe present invention; and

FIG. 4 is a diagram similar to that of FIG. 2, schematicallyillustrating a layout of contiguous cells as present in anotherembodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The manner in which the invention satisfies the stated needs describedearlier will now be explained with respect to each of the problemsituations described in the Background section of this specification.The following description is exemplary in terms of a Private MobileRadio (PMR) system, with base stations having direct connectioncapability. The invention may however be applicable also to other typesof cellular systems, with modifications which will be readily apparentto persons skilled in the art. The method of the invention calls formeasures to be taken on two different occasions—(1) upon setting up andprogramming (or re-programming) a particular cellular system and (2)when a fault is detected. The first mentioned occasion may even extendback to the planning phase of the system, whereby some of itscharacteristics (notably cell boundaries and channel allocation) may bedetermined iteratively with the needs of fallback procedures, especiallywhen operating according to the present invention.

During system setup each cell is classified as to its relative size, itsrole in the cellular scheme (e.g. dominant, fill-in or something betweenthem) and the degree of congruence between the cell coverage and thelikely distribution of subscribers belonging to any one group. Inanticipation of a system-wide station isolation fallback, certain basestations (notably those with wide coverage and high congruence withgroups) are designated as “key stations”. Other base stations, havingless group coverage, are designated as “non-key stations”; these mayinclude all other base stations in the system. Each base station is thenprogrammed, according to its classification and designation, to functionin fallback situations as set forth below. The programming preferablyconsists of storing in the respective base station, as well as in thecentral switch, certain commands and parameters, to be activated orpassed on during a fallback procedure.

The event of a base station becoming isolated from the network is sensedby the affected station, as well as by the rest of the system, and thisinitiates corresponding fallback procedures. The present inventionspecifies that such a procedure include changing the operating power ofthe isolated base station and/or of nearby (usually surrounding)co-channel base stations, according to how they were programmed. In apractical system there may be many considerations brought to bear on theprogram parameters loaded into any base station, as will be discussedfurther below. Typically, the following will be effected: If theisolated station has a large coverage area, which, moreover, includesthe expected mobile subscriber unit locations of a considerableproportion of one group, its allowed power level is preferablyincreased, possibly up to the maximum attainable level. At the sametime, the power level of each nearby co-channel base station ispreferably decreased. The effect of each of these two changes, and ofcourse also of their combination, is to increase the C/I ratio withinthe cell of the isolated base station, as well as to widen its boundary(for any acceptable threshold of C/I). The resultant fallback situationis illustrated in FIG. 3 in the case of an ideal, hexagonal, layout ofcells (similar to that depicted in FIG. 2). Here the isolated cell isoutlined by a heavy line and the widened boundary schematically denotedby a heavy circle; the nearby co-channel base stations, whose power maybe reduced, are marked by dotted fill-patterns. Thus more mobilesubscriber units within the isolated cell are induced to remain linkedto the respective base station, while some mobile subscriber units thatwere formerly linked to an adjacent base station (on another channel)are induced to re-select (switch over to) the isolated base station. Theeffect is that more subscribers, especially those belonging to the groupthat normally populates the respective cell, are able to communicateamong themselves.

Biasing the threshold values in mobile units through specializedprocedures, such as “Home Cell” and “Preferred Locations” to favourcertain cells is known. According to the former procedure, all membersof a group are assigned a particular base station as their “Home Cell”and the reselection parameters are set so as to favour that base stationwhen in its vicinity. According to the latter procedure, the mobileunits of all members of a group are given a list of preferred basestations and, optionally, a list of unfavorable base stations. Theseknown procedures may be used in embodiments of the present invention.

Thus, if the mobile subscriber units of the group that normallypopulates the affected cell are also set to refer to the isolated basestation as their Home Location, which means that their reselectionparameters are biased toward that base station, the positive change inthe C/I, caused by the power changes according to the present invention,enhances the Home Location effect. By such means, the number of mobilesubscriber units potentially interconnected through the isolated basestation is even greater. This is similarly true for the case that theisolated base station is in the list of preferred stations.Alternatively and optionally, the isolated and adjacent co-channel basestations can be programmed to send to the mobile subscriber unitsappropriately modified threshold values for reselection—again achievingthe same enhancement effect.

If the isolated station has mainly a fill-in role, its allowed powerlevel is preferably decreased. At the same time, the power level of eachnearby co-channel base station is preferably increased The effect ofeach of these two changes, and of course also of their combination, isto decrease the C/I ratio within the cell of the isolated base station,as well as to shrink its boundary (for any acceptable threshold of C/I).Thus some mobile subscriber units within the respective cell are inducedto reselect an adjacent base station (co-channel or on another channel),while mobile subscriber units that are linked to an adjacent basestation are induced to remain so over a greater distance therefrom. Theeffect is that more subscribers, especially those belonging to a groupthat normally populates any of the adjacent base stations, are able tocommunicate among themselves. In addition, also more of the othersubscribers within the affected cell can now communicate with the restof the system.

It is to be noted that this beneficial effect will be enhanced if theaffected group of mobile subscriber units also has one of the adjacentstations on its preferred list, or the isolated station on itsleast-preferred list. Alternatively and optionally, the isolated andadjacent base stations can be programmed to send to the mobilesubscriber units appropriately modified threshold values forreselection—again achieving the same enhancement effect.

It will be appreciated that all that was described above for a singleisolated base station is also applicable for the fallback situation ofseveral base stations having become isolated simultaneously. Clearly,some modifications will be necessary if any two of these stations are inmutual proximity.

It is to be noted that the scheme described above has many parameters,which depend on system characteristics that are unique to any onedeployed system and may, moreover, change as the system changes. Thesystem characteristics include base station locations, their maximumpower levels and the topography (all of which determine cell boundariesand overlaps); they also include channel re-use pattern (such asdepicted in FIG. 2 in an ideal case), definitions of subscriber groupsand data about the most likely location regions of their members. Thelatter is referred to as the expected locations of the subscribers or ofthe groups.

Fallback parameters, which are pre-loaded into each base station, aswell as into the central switch, preferably include:

-   -   the command (or lack thereof) to change power in the event of        the base station becoming isolated and the amount and sense        (i.e. an increase or a decrease) of such change—all stored in        each base station;    -   the command (or lack thereof) to change power of a base station        in the event of a nearby co-channel base station becoming        isolated, and the amount and sense of such change—stored in the        central switch or in each base station;    -   the command to increase power in any key station in the event of        massive system related isolation, and the amount of such        increase—stored in the respective key station;    -   the command (or lack thereof) to decrease power in any non-key        station in the event of massive system related isolation, and        the amount of such decrease—stored in the respective non-key        station.

It will be appreciated that such parameters may also take other forms,such that the combination of a command and an amount is represented by asigned change value, where 0 means no change.

As discussed above, these fallback parameters may be determined for eachbase station, based, inter alia, on its classification with respect tocoverage areas and to subscriber groups. However, all systemcharacteristics are brought to bear in optimally determining theparameters, especially the amounts of change. There are, in general, nofirm rules for this optimal determination process, since it depends onthe characteristics in a complex way and each deployed system is unique.Additional considerations that may be brought to bear are the relativeimportance of the various groups of subscribers. It will, however, beappreciated that the principles of the present invention can beadvantageously applied in any case, whereby in a fallback situation thenumber of linkable subscribers is appreciably increased, even if notachieving the optimal service for all subscribers (e.g. maximum numberof linkages). It is to be noted in this context that the term “optimal”has a variable meaning and depends on various preferences andoperational considerations, some of which have statistical bases.

We turn now to the other major fault situation wherein all base stationsbecome simultaneously isolated from the system. As discussed in theBackground section, the main achievable goal for a fallback procedure isto maximize the number of inter-linkable subscribers that belong to anygroup, the assumption being that each group is concentrated in thevicinity of one base station. It has been described above how, accordingto the present invention, certain base stations are designated as keystations and certain others as non-key stations. Further according tothe present invention, some or all key stations are so programmed thatupon the occurrence of such a fault situation, the respective power ofeach is increased. Also according to the present invention, some or allnon-key stations are so programmed that upon the occurrence of such afault situation, the respective power of each is decreased. Generallythere will be such key stations and non-key stations associated witheach set of reusable channels. Preferably key stations and non-keystations associated with any one set of channels are chosen so as to bemutually interleaved; thus usually some co-channel non-key stations willbe nearer to any key station than are other co-channel key stations.

As a result, in an overall fault situation, the C/I ratio within thecell of each key station increases, while its boundary (for anyacceptable threshold of C/I) widens. Thus more mobile subscriber unitswithin each corresponding cell are induced to remain linked to therespective key station, while some mobile subscriber units that wereformerly linked to adjacent base stations (on other channels), whichgenerally are non-key stations, are induced to switch over to that keystation. The effect is that more subscribers belonging to the group thatnormally populates a cell that corresponds to a key station remainconnected and are able to communicate among themselves.

The resultant fallback situation is illustrated in FIG. 4 in the case ofan ideal, hexagonal, layout of cells (similar to that depicted in FIG.2). Here cells that contain key base stations are marked by dottedfill-patterns and their increased coverage areas are schematicallydenoted by heavy circles. The layout of key stations in the diagram ofFIG. 4 also illustrates another preferred feature of the invention,namely that key stations are preferably chosen so that their normal cellcoverage areas are non-contiguous. The latter feature minimizes theeffect of mutually proximate key stations counteracting each other inaffecting the reselection process of mobile subscriber units withintheir overlapping coverage areas. As shown in FIG. 4, the key stationsare the station of a cell operating on channel 1 deemed to be inside theset of key cells and the ring of nearest but not contiguous cells havingstations operating respectively on channels 2 to 7 respectively.

Clearly, in any practical system the station layout pattern and the cellboundaries are generally irregular and more complex. Hence, thedesignation of key stations and the parameters, such as size of powerchange, loaded into each base station for this fallback procedure, aredetermined, again, by complex considerations that are based on thecharacteristics of the particular system and its subscribers. As in thesingle-base station isolation type of fault, discussed above, suchconsiderations may be aimed at optimizing, in some sense, the possibleintra-group communication; however the principles of the methoddisclosed herein can be applied advantageously in most systems evenwithout seeking optimal results.

The known procedures of “Home Cell” and “Preferred Sites” designationand of transmitting new C/I threshold parameters, all mentioned above,can optionally be used to enhance the effect of the method of thisinvention also in the situation of overall base station isolation. Forexample, Home Cell may be identified with key stations and, as a result,corresponding mobile subscriber units would be even further biasedtoward them in a fallback situation. Similar effects can be achieved byPreferred Sites being identified with key stations and by downloadingappropriate reselection parameters, as explained above with respect thesingle-base station isolation fallback situation.

It will be understood that, while the description above is in terms ofmethods, the present invention also includes a cellular communicationsystem programmed to carry out the methods, as well as a base stationfor use in a cellular communication system, being operative, uponbecoming isolated from the system or upon a nearby co-channel basestation having become isolated from the system, to change its ownoperating power, without necessarily switching the power off.

It will likewise be understood that many more configurations of thesystem and method disclosed above, and modifications thereof, arepossible—all coming within the scope of the invention, as defined in theclaims.

1. A method of operation for use in a radio communications systemcomprising a cellular network of base stations and mobile units linkedto such base stations, the method comprising the following steps, withrespect to any base station operable to provide direct radiocommunication with mobile units linked thereto: (i) detecting that abase station has become isolated from the system, wherein said isolatedbase station has lost at least one link to other base stations but isstill in radio communication with its mobile units and, (ii) changing anoutput power level of said isolated base station relative to an outputpower level of one or more selected co-channel base stations ofneighboring cells of the cellular system, whereby the range ofcommunications provided by said isolated base station is changed to adifferent finite value relative to that provided by the said co-channelbase stations.
 2. A method according to claim 1, wherein the system inwhich the method is applied comprises a system in which base stations ofadjacent cells within groups within the network operate at differentfrequencies and operational frequencies are repeated in different groupsof cells within the network and said co-channel base stations are indifferent groups from that including said isolated base station.
 3. Amethod according to claim 1 which comprises changing the output powerlevel of selected co-channel base stations in a set of groups of cellsnearest to the group which includes said isolated base station.
 4. Amethod according to claim 3, wherein the selected co-channel basestations are base stations located in a ring around the isolated basestation and operating on the same frequency as the isolated basestation.
 5. A method according to claim 1, wherein the actual orexpected communication coverage to mobile units provided by the isolatedbase station is such that the isolated base station is designated asfavored compared to said selected co-channel base stations and theoutput power level of the isolated base station is increased in step(ii) thereby to increase the range of communications provided by theisolated base station.
 6. A method according to claim 1, wherein theoperating power level of said selected co-channel base stations isreduced in step (ii).
 7. A method according to claim 5, wherein a basestation selection program is stored within mobile units in a groupwithin communication coverage of at least one of the isolated basestation and said selected co-channel base stations wherein the programis to steer communication of the mobile units to favour the isolatedbase station after isolation has been detected.
 8. A method according toclaim 1, wherein the actual or expected communication coverage to mobileunits provided by the isolated base station is such that the isolatedbase station is designated as not favored compared to said selectedco-channel base stations and the output power level of the isolated basestation is reduced in step (ii).
 9. A method according to claim 1,wherein the output power level of said selected co-channel base stationsis increased in step (ii).
 10. A method according to claim 9, wherein abase station selection program is stored within mobile units in a groupwithin communication coverage of at least one of the isolated basestation and said selected co-channel base stations wherein the programis to steer communication of the mobile units away from the isolatedstation and toward one or more of said selected co-channel base stationsafter isolation has been detected.
 11. A method according to claim 1,wherein a base station selection program is stored within mobile unitsin a group within communication coverage of at least one of the isolatedbase station and said selected co-channel base stations wherein theprogram is to steer communication of the mobile units after isolationhas been detected, wherein the selection program is selected from HomeLocation program and a Preference List program.
 12. A method accordingto claim 1, wherein certain base stations of the system are designatedas key stations and certain other base stations of the system aredesignated as non-key stations, and wherein the method compriseschanging relative to one another the operating power level of saidisolated base station and the output power level of one or more selectedneighboring base stations of the cellular system according to whetherthe isolated base station is a key or non-key station.
 13. A methodaccording to claim 12, which includes decreasing or switching off theoutput power of one or more non-key base stations that have becomeisolated from the system.
 14. A method according to claim 12, whichcomprises maintaining or increasing the output power level of one ormore key base stations that have become isolated from the system.
 15. Amethod according to claim 12, wherein a base station selection programis stored within mobile units in a group within communication coverageof at least one or more isolated key base stations and said selectedco-channel base stations wherein the program is to steer communicationof the mobile units after isolation has been detected.
 16. A methodaccording to claim 15, wherein said mobile units are programmed to havea Home Location program specifying a home location that corresponds toone of said key stations.
 17. A method according to claim 14 and whereinsaid mobile units are programmed to have a Preference List whichincludes therein one or more of said key stations.
 18. A methodaccording to claim 12, wherein said designation of key and non-key basestations is such that for each key station there is at least oneco-channel non-key station located nearer thereto than any otherco-channel key station.
 19. A method according to claim 12, whereinevery base station of the cellular network of the system ispre-designated as either a key station or a non-key station.
 20. Amethod according to claim 1, wherein the output power level of aplurality of selected neighboring base stations is changed relative tothe output power level of other selected neighboring base stations. 21.A method according to claim 20 and wherein the output power level of aset of selected base stations operating in different channels ismaintained or increased and the operating power level of otherneighboring base stations operating in the same channel as any of theselected set is reduced or switched off.
 22. A base station for use in acellular communication system, the base station being operable, upondetecting that it has become isolated from the system or that a nearbyco-channel base station has become isolated from the system, to changeits own operating power level relative to the output power level of atleast one other base station.